1. Field of the Invention
The present invention relates to an optical transmission system and an optical transmission method in which round-trip transmission phase compensation data is acquired at time intervals synchronized with integration timings at a signal transmission destination to perform transmission phase compensation in a postprocessing in a case where an integration processing is performed for a time shorter than several seconds at the signal transmission destination.
2. Description of the Related Art
FIG. 6 illustrates the effect of an improvement in a phase stability of a transmission signal using a transmission phase compensation device in the optical transmission systems disclosed in Japanese Patent Application Laid-open Nos. 2009-060241 and 2011-071700 in the form of a relationship between the measured phase stability and time (for example, 80 GHz and a transmission path length of 10 km).
FIG. 6 uses an internationally-used indicator called “Allan standard deviation” which indicates the phase stability of an atomic frequency standard or the like. In a case of only a fiber cable (black circle marks ●), a flicker frequency noise (the Allan standard deviation value is constant regardless of the lapse of time with a characteristic line being substantially horizontal) appears in 10 seconds or longer, and enters a state in which a phase is unstable. Meanwhile, in a case of using the inventions disclosed in Japanese Patent Application Laid-open Nos. 2009-060241 and 2011-071700 (white square marks □), it is clear that the signal transmission is performed over a long term while suppressing the flicker frequency noise and exhibiting a white phase noise with a stable phase (the Allan standard deviation value is in inverse proportion to time).
Occasions in which a high phase stability is demanded for a transmission signal are classified into a case (1) where real-timeness is required and a case (2) where real-timeness is not required. In general, the case (1) where the real-timeness is required is in great demand. Meanwhile, a signal processing performed by time integration at a signal transmission destination corresponds to the case (2) where the real-timeness is not required. For example, in astronomical observation, observed signals, which are extremely faint, are time-integrated and improved in signal-to-noise ratio, and then necessary data is acquired.
Referring to FIG. 6, it is clear that there is not much difference in the phase stability between use and non-use of a transmission phase compensation device in an integration time shorter than several seconds. In other words, optical fiber cable transmission does not have much influence on the short-term phase stability.
Therefore, in the case (2) where the real-timeness is not required, in particular, in a case where an integration processing is performed at the signal transmission destination for a time shorter than several seconds (approximately three seconds), there is no need to provide a phase-locked loop circuit for performing round-trip transmission phase compensation in real time, a microwave signal phase shifter, or the like of the optical transmission systems disclosed in Japanese Patent Application Laid-open Nos. 2009-060241 and 2011-071700. On the contrary, the phase-locked loop circuit, the microwave signal phase shifter, or the like complicates a system configuration, thereby increasing signal loss, which in turn inhibits the improvement in the phase stability of a transmitted signal. This further leads to a problem that digitization and larger packaging densities become hard to achieve and that a disadvantage is imposed on multipoint transmission compensation.